Britain has seen a sharp increase in the number of people unable to afford to feed themselves at the most basic level, thanks to the worsening economic climate and changes to the benefit system, according to a survey by a leading food charity.

In the past year FareShare, which redistributes waste food from major food manufacturers and supermarkets to social care charities, has seen a 20% rise in the number of people it is feeding – from 29,500 a year to 35,000.

And many of those, blighted by rising unemployment and business failures, are coming from the sorts of stable family backgrounds once considered immune to the worst effects of recession.

The new findings, which are backed up by research from other organisations working in the same field, will make sobering reading for the Conservative party as it gathers in Manchester this weekend for its annual conference, where the direction of the government's stringent deficit reduction programme will be carefully scrutinised.

The number of charities that have signed up to receive food from FareShare, which operates from 17 sites across the UK, has also risen in the past 12 months, from 600 to 700. More than 40% of those charities are recording increases in demand for their feeding services of up to 50%.

"People in our communities are going to bed hungry because they can't afford to feed themselves," said Lindsay Boswell, chief executive of FareShare. "This is a huge problem and it's right here, in our neighbourhoods, on our streets. This is outrageous enough even before you factor in the thousands of tonnes of good food thrown away each year. It's illogical and frankly immoral that these problems coexist."

The food that FareShare distributes would generally end up in landfill sites. It is discarded by major supermarkets such as Sainsbury's, Tesco and M&S, because it's out of date, or surplus to demand or as a result of printing errors on the packaging.

It's estimated that three million tonnes of food like this is being wasted in Britain every year, of which FareShare gets hold of about 1%. "Demand for our food is going up far faster than we can source it," Boswell said. "As a charity we started out purely interested in liberating waste. We are an environmental charity that gets bloody angry about food being thrown away. However, we're clear that it is the alleviation of poverty which now leads what we do."

One of the major changes seen by FareShare and organisations like it is in the type of people they are now feeding. Where once it was single homeless and the chronically destitute now it's increasingly families and working people who have fallen on hard times.

In the past year, the Salisbury-based Trussell Trust has seen the number of people it is feeding rise from 41,000 to 61,500. It runs more than 100 food banks around the country, distributing emergency food parcels to people in dire need who have been referred to it by social care organisations and charities.

"We're seeing a big increase in what you could call, for want of a better phrase, normal working people, those who have lost their jobs or seen their own businesses go under," says Jeremy Ravn, manager of the food bank network. "The big problem is that the welfare state is not reacting fast enough to need."

An increasing time lag between benefits claims being accepted and the date when payments come on stream is, Ravn says, resulting in some people suffering serious hunger.

A spokesperson for the Department of Work and Pensions denied there had been any changes to the system for paying benefits which could be blamed for the sharp increase in the number of people requiring food aid.

However he said that a series of reforms, including the controversial plan by work and pensions secretary Iain Duncan Smith for a universal credit to replace a slate of existing benefits, was now more necessary. "This will help us get back to a working welfare state where people don't have to rely on food parcels," the spokesperson said.

• More than one in five workers now earn less than a "living wage", says the Resolution Foundation thinktank. Its head, Gavin Kelly, said the research showed how pervasive low pay is.